1. Field of the Invention
The present invention relates to the production of synthetic scheelite, i.e., calcium tungstate (CaWO.sub.4), from aqueous solutions of sodium tungstate (Na.sub.2 WO.sub.4). More particularly, the present invention concerns the treatment of sodium tungstate solutions containing significant amounts of sodium sulfate dissolved therein. Such solutions are typically produced in tungsten refining operations by digestion of tungsten-bearing ore with an alkaline solution to form a raw liquor that is in turn subjected to a series of purification steps to remove contaminants that enter solution with the tungsten values. Substantial quantities of sulfate ion are introduced into the tungstate liquor through the initial digestion step and through the addition of sulfate-bearing reagents in subsequent purification steps.
It is desirable to be able to recover calcium tungstate from sodium tungstate liquors for several reasons. Calcium tungstate is a valuable commercial product in its own right, and is also used to upgrade artificially the tungsten content of the feed to a tungsten refinery. Formation of calcium tungstate from low-grade tungsten concentrates and dilute tungstate liquors that are not readily upgraded or that can be upgraded only with a significant loss of tungsten provides the tungsten values economically in a useful form.
To be a valuable commercial product for use in alloying of steel, the calcium tungstate produced from tungstate liquors must be low in sulfur; therefore, it is necessary to avoid recovering calcium sulfate in the calcium tungstate product. Presently known techniques for avoiding sulfate contamination while producing calcium tungstate typically produce chloride and sulfurous byproducts that need to be disposed of, and are also relatively difficult to control for attaining optimum calcium tungstate production at satisfactory yields with minimum sulfate contamination. Thus, there is a need for a process for producing high-purity, low-sulfur calcium tungstate from sulfate-bearing feed materials which does not produce undesirable byproducts, which is relatively easy to control, and which does not require execessive amounts of additional reagents.
2. Description of the Prior Art
The conventional method of precipitating calcium tungstate from an aqueous solution of sodium tungstate which also contains sodium sulfate is adding calcium chloride. The presence of sodium sulfate in the solution inevitably results in co-precipitation of calcium sulfate, thereby contaminating the calcium tungstate precipitate. The precipitate is therefore typically calcined with carbon at about 950.degree. C. to reduce the calcium sulfate to calcium oxide plus a sulfur dioxide offgas. This process is unsatisfactory because it consumes large quantities of calcium chloride, and because it generates large volumes sodium chloride solution and sulfur dioxide offgas which must be disposed of. In addition, the calcium oxide byproduct dilutes the calcium tungstate product and lowers the WO.sub.3 content thereof.
In one version of this process which has been published, sodium hydroxide and calcium chloride are added to an aqueous solution of sodium tungstate and sodium sulfate to precipitate about 97 percent of the tungsten in solution as calcium tungstate. The yield in this step is intentionally sacrificed in order to avoid contamination of the product with calcium sulfate. The solids are filtered off, and a slight excess of calcium chloride is then added to the filtrate to precipitate calcium tungstate and calcium sulfate. This second precipitate is recovered, and then digested in a sodium carbonate solution to convert the calcium sulfate into sodium sulfate thereby providing additional calcium tungstate product which is combined with the solid product of the first precipitation step for further treatment. This process is undesirable because it is cumbersome, it consumes large quantities of calcium chloride and sodium carbonate, and it generates a sodium chloride solution which must be disposed of.
U.S. Pat. No. 2,388,306 discloses precipitating calcium tungstate from a liquor containing sodium tungstate and sodium sulfate by adding lime to the liquor. In this process both the yield and the tungstate content of the precipitated product fall short of desirable values. The reaction of the lime with sodium sulfate produces sodium hydroxide in the liquor, thereby raising the pH of the liquor to values greater than 12, unacceptably increasing the amount of tungstate which remains in solution. In addition, the calcium tungstate product is typically diluted with unreacted lime; the patent disclosure reports WO.sub.3 contents of about 65.8 percent and 61.1 percent, which fall short of the theoretical maximum of about 80.5 percent.